Safety lock assembly

ABSTRACT

A safety lock assembly for locking together in a predetermined relative position two members which are arranged for movement relative to one another, displacement means being provided for moving the members relative to one another out of the predetermined position and the safety lock assembly being normally maintained in a locking position and being arranged for movement out of the locking position under the influence of driving means, the driving means and displacement means being connected to a common power source and the driving means being actuated when the power from the source is lower than that required for actuating the displacement means.

United States Patent 1 Lynde 1 1 SAFETY LOCK ASSEMBLY [76] Inventor:Edward A. Lynde, Jordan Valley,

Oreg. 97910 22 Filed: Sept. 29, 1971 21 Appl. No.: 184,865

Related US. Application Data [62] Division of Ser. No. 56,843, July 21,1970.

FOREIGN PATENTS OR APPLICATIONS Germany 292/144 3,740,090 1June 19, 1973Primary Examiner-Leo Fria glia Assistant Examiner-John A. PekarAtt0rneySpencer & Kaye [57] ABSTRACT A safety lock assembly for lockingtogether in a predetermined relative position two members which arearranged for movement relative to one another, displacement means beingprovided for moving the members relative to one another out of thepredetermined position and the safety lock assembly being normallymaintained in a locking position and being arranged for movement out ofthe locking position under the influence of driving means, the drivingmeans and displacement means being connected to a common power sourceand the driving means being actuated when the power from the source islower than that required for actuating the displacement means.

9 Claims, 6 Drawing Figures FIG. 1

PATENIEB Jim 9975 SHEU 1 BF 2 ATTORNEY 5.

FIG. 2

PATENTED JUN 1 9 I975 SHEET 2 BF 2 ATTQRNEYs.

SAFETY LOCK ASSEMBLY CROSS REFERENCE TO RELATED APPLICATION Thisapplication is a division of application Ser. No. 56,843, filed July 21,1970.

BACKGROUND OF TlHlE INVENTION The present invention relates to safetylock assemblies,-particularly for locking a first member of a vehicle ina predetermined position with respect to a second member thereof.

There exist many types of industrial, construction and commercialapparatus and vehicles whose operation involves such a relativemovement. Typical examples are dump bed trucks in which the bed is to beraised relative to the truck frame and tilt cab trucks in which the cabis to be pivoted relative to the truck frame. These raising operationsare to be performed only at certain times and it is essential that suchmovement not occur accidentally or inadvertently because this could leadto serious injury to the operating personnel or innocent bystanders orsubstantial damage to the raising and lowering mechanism or the contentsof the vehicle.

In order to prevent such accidental movements, it has heretofore beenthe practice to provide such apparatus or vehicleswith manually operatedmechanical locks which must be released before the desired movement canoccur. While these locks provide the desired protection againstaccidents as long as they are correctly used, it is highly possibly, andin fact not uncommon, for the operator to forget to secure the lockafter the movable member has been returned to its normal position or toforget to release the lock before operating the mechanism which movesthe member to its raised position. In the former case, there exists theobvious danger that the movable member will become accidentally raisedwhen the vehicle is in motion or when it is otherwise desired that themember not be raised, while in the latter case activation of the raisingmechanism will very likely result in serious damage to the truck bed,the frame, or the raising mechanism itself.

In addition, the need for manually operating the lock mechanismincreases the total time required for raising and lowering the movablemember.

Moreover, in the known safety locks, there is no provision for adjustingthe position of the locking elements relative to the member beingsecured. As a result, when the vehicle is in motion, the movable memberwill experience a certain degree of bounce and twist and apply torquewhich can result in frame fatigue and unnecessary wear on the variouselements on the raising mechanism.

SUMMARY OF THE INVENTION It is a primary object of the present inventionto overcome the above-noted drawbacks and difficulties.

Another object of the invention is to eliminate the need for manuallyoperating a safety lock assembly.

A more specific object of the invention is to automat ically control thesafety lock assembly so that it will alwaysrbe in the desired position.

Yet another object of the invention is to prevent the safety lock fromaccidentally becoming unlocked even if the active member for operatingthe lock should malfunction.

Still another object of the invention is to facilitate the adjustment ofthe safety lock to cause it to be positioned properly with respect tothe member being secured.

These and other objects according to the invention are achieved by theprovision of a safety lock assembly including a locking bolt mounted formovement between a locking position and an unlocking position, passivebiassing means connected for normally urging the bolt into its lockingposition and active driving means connected for moving the locking boltinto its unlocking position. According to a preferred embodiment of theinvention, the active driving means receives its driving power from thesame source as the mechanism for raising and lowering the member beingsecured and is arranged to be actuated when the power from the source islower than that required for initiating the raising of the member. Thisassures that the locking bolt always becomes automatically unlockedbefore the raising of the movable member commences and will not returnto its locking position until the movable member has been returned toits lowered position.

According to another principal feature of the present invention, aresilient piece is interposed between the two members which are to moverelative to one another and the movable member is provided with alocking bolt engaging element which cooperates with the locking bolt soas to force the movable member toward the other member so as to compressthe resilient piece as the bolt moves into its locking position. Thisassures that the members will be connected together in a manner whichprevents relative movement, and particularly vibrations, therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view, partlyin cross section, of a first preferred embodiment of the invention.

FIG. 2 is a plan view taken along the line 22 of FIG. 1.

FIG. 3 is a cross-sectional detail view taken along the line 3-3 of FIG.1.

FIG. 4 is an elevational view, partly in cross section, of a secondembodiment of the invention with the locking bolt in its unlockingposition.

FIG. 5 is a view similar to that of FIG. 4 showing the locking bolt inits locking position.

FIG. 6 is a side view of the embodiment of FIGS. 4 and 5 with thelocking bolt in its locking position.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1, 2 and 3 illustrate onepreferred embodiment of the safety lock assembly according to theinvention for locking a pivotally mounted b ed, such as a dump bed, inits lowered position on a truck or other vehicle. The safety lockassembly is mounted on the frame 1 of the truck or other vehicle bymeans of two vertical plates 3 and 4 welded to truck frame members atrespectively opposite sides of the truck.

The base of the lock assembly itself is composed of two right-anglechannel members '6 and 7 each bolted to a respective one of the verticalplates 3 and 4.

On the upper surface of channel member 6 there is disposed a guideassembly composed of a support base 11 carrying a guide tube 13 andareinforcing tube 14 surrounding the outer end of guide tube 13. Tubes 13and 14 may be welded or otherwise permanently secured to base 11.Reinforcing tube 14 carries an upstanding ear 16 provided with anopening for holding one end of a first tension spring 18. If necessary,flat shims 19 can be disposed between the support base 11 and the uppersurface of channel member 6 for adjusting the height of the tubes 13 and14.

A first locking bolt 21 is mounted to extend through guide tube 13. Bolt21 is dimensioned to be closely matched to the internal lateraldimensions of tube 13 so as to experience a minimum of lateral play withrespect thereto. At its inner end, with respect to the sides of thetruck, bolt 21 carries an upstanding connecting element 23 which may bewelded to, or integral with, bolt 21. Connecting element 23 is providedwith an car 25 having an opening in which is held the other end oftension spring 18.

Mounted on the upper surface of the other channel member 7 is a secondguide assembly substantially identical with the previously-describedguide assembly and composed of a support base 31 carrying a guide tube33 and a reinforcing tube 34. Tubes 33 and 34 are mounted on the supportbase 31 in the same manner as the tubes 13 and 14 of thepreviously-described guide assembly. The support base 31 is rigidlyattached to channel member 7 by means of bolts and between support base31 and channel 7 there can be provided one or more shims 39 foradjusting the height of the tubes 33 and 34.

At the upper surface of reinforcing tube 34 there is disposed an ear 36provided with an opening for supporting one end of a second tensionspring 38.

A second locking bolt 41 extends through tube 33. While bolt 41 isslidable in tube 33, the bolt is dimensioned to closely match theinternal dimensions of tube 33 so as to minimize lateral play betweenthe bolt and the tube.

At its inner end, with respect to the sides of the truck, bolt 41carries an upstanding ear 45 provided with an opening for supporting theother end of tension spring 38. Also at its inner end, bolt 41 carries aprojection 46 extending downwardly from its lower surface. Both ear 45and projection 46 may be integral with, or welded to, bolt 41 or may berigidly attached thereto in any other suitable manner.

Rigidly mounted on the inner vertical surface of channel member 7 is thecylinder 52 of a hydraulic ram unit 51. This cylinder can be rigidlyattached to channel member 7 in any suitable manner, as by bolting orwelding, or by means of a suitable auxiliary support unit rigidlyconnected to channel member 7 and holding the cylinder 52 in such amanner as to permit the cylinder to be removed for maintenance orreplacement. The cylinder 52 is provided with an inlet 53 for deliveringpressure fluid to the interior of the cylinder, the inlet 53 extendingthrough the side of channel member 7 and being readily accessible forthe attachment of a tube for supplying fluid to the interior of thecylinder.

Within cylinder 52 there is disposed the usual piston rigidly connectedto a piston rod 55 projecting out of the inner end of the cylinder. Theram unit 51 could be of any commercially available type and, for a lockassembly to be used on a medium sized dump bed truck, could be of thetype which producesa force of the order of 1,250 pounds and which isprovided with a piston having a two-inch travel.

Piston rod 55 is provided with a threaded end portion 56 connected tothe base of a clevis 58 by means of a bolt forming a part of the clevisand threadedly engaging the threaded end portion 56 to permit adjustmentof the position of the clevis 58 in the direction of the axis of pistonrod 55. The bolt connected to projection 56 is preferably permanentlyattached to the base of the clevis in such a manner as to be rotatabletherewith. Alternatively, the bolt could be stationary with respect tothe clevis while the piston rod 55 is rotatable for adjusting thelocation of the clevis relative to the piston rod.

The projection 46 extending from locking bolt 41 ex tends downwardlythrough the clevis 58, between its legs, and the position of projection46 relative to the length of clevis 58 is fixed by two guide pieces 59and 60. This arrangement permits projection 46 to move vertically withrespect to clevis 58, to allow the vertical position of the locking bolt41 to be varied by changing the number or thickness of the shims 39disposed between channel member 7 and support base 31, while assuringthat projection 46, and thus locking bolt 41 and ear 45, will move as aunit with clevis 58 in the direction of the longitudinal axis of theclevis-piston rod unit.

The ends of the clevis opposite the hydraulic ram unit 51 are providedwith openings for receiving a pivot pin 62 whose end is provided with ahole through which is inserted a cotter pin for holding pin 62 in place.

One end of a lever bar 64 is pivotally mounted on pin 62 so as to renderthe bar pivotal with respect to clevis 58. Bar 64 is pivotally mountedat its center on the inner extremity of channel member 6 by means of apivot bolt 63. The upper surface of channel member 6 is provided with acut-out for the passage of bar 64.

Connected between bar 64 and connecting element 23 of locking bolt 21 isa connecting rod 68 having connecting clevises 70 and 71 connected toits ends. Rod 68 has one end threaded to mate with a bolt forming partof the clevis 70, the bolt preferably being rotatable with respect tothe clevis. This permits the effective length of rod 68 to be adjusted.Clevis 70 is pivotally connected to the other end of bar 64 by means ofa pivot pin 66 held in place by an associated cotter pin. Clevis 71 atthe other end of rod 68 can be integral with or welded to rod 68, orotherwise permanently attached, and is pivotally connected to connectingelement 23 of locking bolt 21 by means of a pivot pin 73 held in placeby an associated cotter pin.

Bar 64 is preferably mounted on channel member 6 so that pins 62 and 66are an equal distance from pivot bolt 63. This assures that lockingbolts 21 and 41 will always travel equal distances when the safety lockis released. The locking bolts 21 and 41 are disposed to extend abovewedge washers 75 and 76, respectively, each permanently attached to thelower flange of a respective one of the side frame members 78 of thedump bed, for preventing the bed from being accidentally raised.

In FIGS. 1, 2 and 3 the safety lock assembly is shown in its fullylocking position with the locking bolts 21 and 41 extending outwardlytoward the sides of the truck and covering substantial portions of thewashers 75 and 76, respectively. The locking position of bolts 21 and 41is determined either by the abutment of connecting element 23 againstthe inner end of guide tube 13 or by the springs 18 and 38 being fullycompressed.

The locking bolts 21 and 41 are normally held in the illustratedposition by the action of tension springs 18 and 38. If desired, thespring 18 could be eliminated and spring 38 would be sufficient to holdboth locking bolts in their closed position and to move the lockingbolts to this position after the fluid pressure in cylinder 52 hasdropped to a low value. When a ram unit 51 of the type described aboveis employed, the spring 38 could be of a type producing a fifty poundpull. Spring 18, if employed, could have a lower pulling force. Whenspring 18 is omitted, spring 38 will act on bolt 21 through the linkagecomposed of projection 46, clevis 58, bar 64, rod 68 and connectingelement 23.

As long as the dump bed remains in its lowered position, as illustrated,locking bolts 21 and 41 do not contact washers 75 and 76, respectively.The necessary shims l9 and 39 can be disposed between bases 11 and 31,respectively, and their associated channel members to establish thisrelationship. This assures that when the bed is lowered, no resistancewill be offered to the movement of the locking bolt into the lockingposition illustrated, or to the movement of the bolts out of thislocking position preliminary to the raising of the dump bed. However,the locking bolts will prevent the dump bed from accidentally movinginto its raised position.

The fluid inlet 53 for the cylinder 52 is connected to a fluid pressuresource 93 by line 97. It is most preferable that the inlet 53 beconnected to the source of fluid pressure for the hydraulic system 95provided for actually raising and lowering the dump bed. The rain unit51 is designed to drive its associated piston, and hence piston rod 55,when the fluid pressure within the cylinder 52 reaches a value lowerthan that required for initiating the raising of the dump bed.Correspondingly, the ram unit 51 is designed so that springs 18 and 38,or spring 38 alone, will overcome the force produced by the ram when thefluid pressure in the cylinder is lower than the hydraulic pressurevalue at which the previously raised dump bed will return to its loweredposition, which is the position illustrated in FIGS. 1-3. This meansthat the ram unit 51 will act, by displacing clevis 58, to move lockingbolts 21 and 41 into their retracted positions, where they are clear ofwashers 75 and 76 and frame members 78 before the hydraulic systembegins to raise the dump bed and that the locking bolts 21 and 41 willbe permitted to return to their locking position only after the dump bedhas been fully lowered.

As a result, it is not necessary to perform a separate, manual,operation to release the safety lock assembly or to subsequently returnthe assembly to its locking position.

Moreover, the assembly requires a positive hydraulic pressure to moveinto its release position. As a result, any malfunctions developing inthe cylinder or breaks in the line delivering fluid to the cylinder willcause the assembly to remain in its locked position rather than in itsreleased position.

Assuming that the fluid inlet 53 for the ram unit 51 is connected to thehydraulic power source for lifting and lowering the dump bed, when it isdesired to lift the bed from the position illustrated, fluid isdelivered to both unit 51 and the system 95 for raising and loweringbed. The relation between the driving force of ram unit 51 and therestraining, or locking, force of springs 18 and 38 is such that the ramunit will generate sufficient force to drive piston rod 55 against thespring restraining force when the fluid pressure in unit 51 is lowerthan that required to begin raising the dump bed. Therefore, at thestart of application of fluid pressure, and before the bed has begun torise, piston rod 55 is driven to the left with respect to the views ofFIGS. 1-3, carrying clevis 58 along with it. Projection 46 and lockingbolt 41 move horizontally with clevis 58 to withdraw locking bolt 41from above washer 76. At the same time, the movement of clevis 58 pivotsbar '64 clockwise with respect to the view of FIG. 1, thus moving rod 68to the right together with element 23 and locking bolt 21. This causeslocking bolt 21 to move free of washer 75. The dump bed is now unlocked.The pressure produced by the fluid source continues to rise, thusactivating the system for raising the dump bed. As long as the fluidpressure is sufficient to raise the dump bed, or to maintain it in itsraised position, the locking assembly with remain in its activated, orreleased state.

When it is desired to lower the dump bed, the fluid pressure graduallydecreases, causing the bed to be lowered. Because of the relationshipsexisting between the system for raising and lowering the bed and thelocking assembly, the pressure delivered to the bed drive system willdrop to a value at which the bed becomes fully lowered before thepressure in the cylinder 52 reaches a value at which the force producedby springs 18 and 38 overcomes the driving force applied to the pistonrod 55. When, after the bed has been fully lowered, the pressure incylinder 52 drops below such value, springs 18 and 38 become effectiveto move locking bolts 21 and 41 back into their locking position. Thesystem is now automatically locked and the bed will be prevented frombeing accidentally raised until it is again desired to perform a raisingoperation.

FIG. 1 illustrates one possible structural arrangement for the hydraulicsystems utilized. According to this arrangement, a hydraulic liftingdevice 95 is connected between frame 1 and the dump bed frame 78 forproducing the desired movement the-rebetween. The hydraulic inlet fordevice 95 is connected by a suitable conduit to the fluid pressuresource 93. This source is also connected by a suitable conduit to theinlet 53 of ram 51. As noted above, the pressures delivered to units 95and 51 can be readily adjusted to assure that bolts 21 and 41 will beretracted before the dump bed begins to be raised and that the boltswill not return to their locking position until after the dump bed hasbeen fully lowered.

Turning now to FIGS. 4-6, there is illustrated another utilization ofthe safety lock assembly according to the invention. In this embodiment,the safety lock assembly is employed for securing the cab of a tilt cabtruck in its lowered position. The safety lock assembly itself isidentical with that illustrated in FIGS. 1-3, with the exception thatthe outer extremity of each of the locking bolts tapers outwardly fromits lower surface to its upper surface, as illustrated for the surface22 of locking bolts 21 in FIGS. 4 and 5. The operation of the lockingbolt assembly is identical with that of FIGS. 1-3.

The arrangement for securing a tilt cab includes a lower mount 81rigidly secured to the truck frame 1. The upper surface of mount 81 isshown to have a V- shaped configuration, although other configurationscould also be employed. Permanently attached to the upper surface ofmount 81 is a rubber bushing 83. A cab mount 85 rigidly secured to thetilt cab has its lower surface formed to mate with the bushing 83, whoseupper surface conforms substantially with the upper surface of mount 81.Mount 85 carries a freely rotatable roller 87 arranged to engage withlocking bolt 21'.

An arrangement identical to the elements 81, 83, 85 and 87 is disposedat the other side of the truck to cooperate with a second locking boltcorresponding to locking bolt 41 and having a tapered end comparable tothat of bolt 21'. The cab is mounted on the truck to tilt about ahorizontal axis parallel to the plane of the views of FIGS. 4 and 5 andperpendicular to the plane of the view of FIG. 6.

When the cab is in its lowered position and the locking bolts of thelocking assembly are in their retracted position, as illustrated in FIG.4, cab mount 85 rests on bushing 83. At this time, the mount 85 actuallycontacts the bushing, although a small space is shown between the mountand the bushing in FIG. 4 for the sake of clarity. When the mount 85rests on bushing 83 and bolt 21 is in the retracted position illustratedin FIG. 4, the plane 91 defining the lower surface of bolt 21' isslightly below the highest point of roller 87. For example, in the casewhere the bushing has a thickness of about one inch, the plane 91 can beapproximately onequarter inch below the highest point on the roller.

When the bolt 21' is then moved toward the locking position illustratedin FIG. 5, tapered end surfaced 22 first comes into contact with roller87. As the bolt moves to its locking position, first the lower part ofextremity 22 and then the lower horizontal surface of bolt 21' ride onroller 87, thus forcing mount 85 downwardly and compressing bushing 83.V

Because the mount 85 is being positively forced against lower mount 81and rubber bushing 83 is being compressed, a solid connection betweenthe cab and the truck is assured and vibrations between the mounts isprevented. The degree to which the rubber bushing 83 is compressed canbe readily adjusted by a proper selection of the total thickness of theshims 19. While the bolt 21 is performing the above-described operation,the corresponding bolt at the other side of the locking assembly isperforming an identical operation with respect to the mounts and rubberbushing disposed at the other side of the truck.

The illustrated arrangement has advantages similar to those describedabove in that it provides a secure, positive lock for the tilt cab and,when the fluid inlet for the locking assembly cylinder is connected tothe hydraulic system for raising and lowering the tilt cab, in thosecases where the cab is provided with such an arrangement, a high degreeof safety against accidental tilting of the cab is produced and the needfor a separate manual operation to lock and unlock the cab iseliminated.

While the mounts 81 and 85 are shown to be provided with V-shaped matingsurfaces, it will be appreciated that these surfaces can have any otherdesired configuration. However, a configuration similar to thatillustrated has the advantage of assuring a proper positioning of thecab relative to the truck. The operation of the locking assembly in thearrangement illustrated in FIGS. 4-6 is identical with that previouslydescribed with reference to FIGS. l-3.

It will be readily appreciated that while the locking assembly accordingto the present invention has been shown in association with certainvehicle structures, the safety lock assembly according to the inventionis readily adaptable to many other vehicle-mounted and stationarystructures. In general, the assembly can be used for locking any unitwhich is arranged to be selectively movable with respect to anotherunit. However, the safety lock assembly according to the invention isparticularly suited for locking movable structures which arehydraulically or pneumatically driven. In addition, because of the highreliability and light weight of the safety lock assembly according tothe invention it is particularly well suited for use in vehicles, wherelight weight is of great importance.

It will be understood that the above description of the presentinvention is susceptible to various modifications, changes andadaptations, and the same are intended to be comprehended within themeaning and range of equivalents of the appended claims.

I claim:

1. For locking a tilt mounted cab which is movable with respect to atruck frame on which said cab is mounted for pivotal movement relativethereto, a safety lock assembly mounted for locking said cab in apredetermined position relative to said truck frame, said assemblycomprising: support means rigidly connectable to said truck frame; alocking bolt mounted on said support means for movement relative theretobetween a first position in which it locks said cab in saidpredetermined position and a second position in which it offers noimpediment to movement of said cab away from said predeterminedposition; passive biasing means connected between said support means andsaid locking bolt for normally urging said bolt into its said firstposition; active driving means connected to said locking bolt andarranged to be actuated for overcoming the biasing force produced bysaid biasing means so as to move said locking bolt into its said secondposition; a first mount rigidly connected to said truck frame; a secondmount rigidly connected to said cab to be disposed directly above saidfirst mount; a resilient, elastic bushing permanently connected to oneof said mounts and arranged to contact the other of said mounts whensaid cab is in its lowered position; and bolt engaging means mounted onone said mount and having a portion disposed to lie in the path oftravel of said locking bolt when said cab is in its lowered position andsaid bolt is in its said second position and said bolt being arrangedfor deflecting said engaging means so as to compress said bushingbetween said mounts as said bolt moves into its said first position.

2. An arrangement as defined in claim 1 wherein there are two lockingbolts, each disposed to a respec tive side of said truck frame.

3. An arrangement as defined in claim 2 wherein said driving meanscomprises a linkage arrangement for moving said bolts into their saidsecond position by moving them toward one another and for permittingsaid bolts to move into their said first position by moving away fromone another under the influence of said biassing means.

4. An arrangement as defined in claim 3 wherein said linkage arrangementcomprises a first linkage for moving one said bolt in the direction ofthe movement produced by said driving means and a second linkage formoving the other said bolt in the direction opposite to the direction ofmovement produced by said driving means.

5. An arrangement as defined in claim 4 wherein said first linkagecomprises means for permitting the position of said one bolt to beadjusted in a direction normal to the direction of the movement producedby said driving means.

6. An arrangement as defined in claim I wherein said driving means arecomposed of a fluid operated ram unit having a driven member connectedto said bolt, and further comprising: fluid operated displacement meansconnected between said tilt mounted cab and truck frame for producingreiative movement therebetween; and a fluid pressure source connected tosaid displacement means for controlling the movement produced therebyand connected to said ram unit for driving said driven member.

7. An arrangement as defined in claim 6 wherein said driven member ofsaid ram unit is arranged to overcome the biassing force of saidbiassing means when the fluid pressure from said source is lower thanthat 10 required for actuating said displacement means.

8. An arrangement as defined in claim 1 further comprising: movementproducing means connected between said tilt mounted cab and said truckframe; a driving power source connected to said driving means forproducing a first force sufficient to move said locking bolt into itssaid second position and connected to said movement producing means forproducing a sec- 0nd force sufficient to move said first member awayfrom said predetermined position, the power from said source requiredfor producing said first force being lower than that for producing saidsecond force.

9. An arrangement as defined in claim 1 wherein said support means arerigidly connectable to said 1 truck frame.

1. For locking a tilt mounted cab which is movable with respect to atruck frame on which said cab is mounted for pivotal movement relativethereto, a safety lock assembly mounted for locking said cab in apredetermined position relative to said truck frame, said assemblycomprising: support means rigidly connectable to said truck frame; alocking bolt mounted on said support means for movement relative theretobetween a first position in which it locks said cab in saidpredetermined position and a second position in which it offers noimpediment to movement of said cab away from said predeterminedposition; passive biasing means connected between said support means andsaid locking bolt for normally urging said bolt into its said firstposition; active driving means connected to said locking bolt andarranged to be actuated for overcoming the biasing force produced bysaid biasing means so as to move said locking bolt into its said secondposition; a first mount rigidly connected to said truck frame; a secondmount rigidly connected to said cab to be disposed directly above saidfirst mount; a resilient, elastic bushing permanently connected to oneof said mounts and arranged to contact the other of said mounts whensaid cab is in its lowered position; and bolt engaging means mounted onone said mount and having a portion disposed to lie in the path oftravel of said locking bolt when said cab is in its lowered position andsaid bolt is in its said second position and said bolt being arrangedfor deflecting said engaging means so as to compress said bushingbetween said mounts as said bolt moves into its said first position. 2.An arrangement as defined in claim 1 wherein there are two lockingbolts, each disposed to a respective side of said truck frame.
 3. Anarrangement as defined in claim 2 wherein said driving means comprises alinkage arrangement for moving said bolts into their said secondposition by moving them toward one another and for permitting said boltsto move into their said first position by moving awaY from one anotherunder the influence of said biassing means.
 4. An arrangement as definedin claim 3 wherein said linkage arrangement comprises a first linkagefor moving one said bolt in the direction of the movement produced bysaid driving means and a second linkage for moving the other said boltin the direction opposite to the direction of movement produced by saiddriving means.
 5. An arrangement as defined in claim 4 wherein saidfirst linkage comprises means for permitting the position of said onebolt to be adjusted in a direction normal to the direction of themovement produced by said driving means.
 6. An arrangement as defined inclaim 1 wherein said driving means are composed of a fluid operated ramunit having a driven member connected to said bolt, and furthercomprising: fluid operated displacement means connected between saidtilt mounted cab and truck frame for producing relative movementtherebetween; and a fluid pressure source connected to said displacementmeans for controlling the movement produced thereby and connected tosaid ram unit for driving said driven member.
 7. An arrangement asdefined in claim 6 wherein said driven member of said ram unit isarranged to overcome the biassing force of said biassing means when thefluid pressure from said source is lower than that required foractuating said displacement means.
 8. An arrangement as defined in claim1 further comprising: movement producing means connected between saidtilt mounted cab and said truck frame; a driving power source connectedto said driving means for producing a first force sufficient to movesaid locking bolt into its said second position and connected to saidmovement producing means for producing a second force sufficient to movesaid first member away from said predetermined position, the power fromsaid source required for producing said first force being lower thanthat for producing said second force.
 9. An arrangement as defined inclaim 1 wherein said support means are rigidly connectable to said truckframe.